Viperidae, whose members are commonly known as vipers, is a family of venomous snakes characterized by a head that is distinct from the body and with a single pair of long, hollow, venom-injecting fangs that can be folded back against the top of the mouth, tip inward, when the mouth is closed. The term viperid also is used for members of Viperidae and offers greater specificity than the term viper in distinguishing this family from its subfamily Viperinae, whose members technically may be known as viperines. Although some authorities use viper for members of Viperinae and pit viper for members of the subfamily Crotalinae (Nilson and Gutverlet 2004), the use of viper in this article will be restricted to members of Viperidae. There are about 250 species of viperids.

Viperids are found all over the world, except in Australia and Madagascar. All have relatively long hinged fangs that permit deep penetration and injection of venom. Four subfamilies are currently recognized: Crotalinae, Viperinae, Causinae, and Azemiopinae (Nilson and Gutverlet 2004; ITIS 2004). Well-known members of Viperidae include such pit vipers (with heat-sensing pits) as rattlesnakes and moccasins in the subfamily Crotalinae and such true or pitless vipers as puff adders in the subfamily Viperinae. Azemiopinae has a single genus and species (Azemiops feae) and Causinae has a single genus, Causus, with six species.

Although vipers often inspire loathing and fear, humans are not their prey, and the rare venomous attacks usually are the result of carelessness or aggression on the part of people. Rather, vipers play a key ecological role in helping to control prey species, including agricultural and household pests such as mice and rats.

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Description

Almost all vipers have keeled scales, a stocky build with a short tail, and, due to the location of the venom glands, a triangular-shaped head distinct from the neck. Their eyes have vertically elliptical, or slit-shaped, pupils that can open wide to cover most of the eye or close almost completely, which helps them to see in a wide range of light levels.

Vipers exhibit great variety in size, shape, and color. They range in size from 30 centimeters (11.8 inches) in the dwarf puff adder to 3.6 meters (11.8 feet) in some pit vipers of the genus Lachesis (Nilson and Gutberlet 2004). The tail can be very short, such as in rattlesnakes, or relatively long in arboreal species, including prehensile in some species adapted fully to an arboreal life style (Nilson and Gutberlet 2004). Pit vipers are recognized by their distinctive, heat-sensitive pits, found on both sides of the head, between the nostril and eye (Nilson and Gutberlet 2004). Members of the pit viper genera Crotalus and Sistrurus are known for the rattle at the end of the tail, which consists of a series of horny, hollow segments of skin, loosely attached, that can be vibrated or shaken to make a rattling or buzzing sound.

All viperids have a pair of relatively long solenoglyphous (hollow) fangs, that are used to inject venom from glands located towards the rear of the upper jaws. Each of the two fangs is at the front of the mouth on a short maxillary bone that can rotate back and forth. When not in use, the fangs fold back against the roof of the mouth and are enclosed in a membranous sheath. The left and right fangs can be rotated together or independently. During a strike, the mouth can open nearly 180° and the maxilla rotates forward, erecting the fangs as late as possible so as the fangs do not become damaged. The jaws close on impact and powerful muscles that surround the venom glands contract to inject the venom as the fangs penetrate. This action is very fast; in defensive strikes it can be more a stab than a bite. Viperids use this mechanism primarily for immobilization and digestion of prey. Secondarily it is used for self-defense. In most cases with non-prey items, such as humans, vipers are more likely to give a dry bite (not inject any venom).

The ability to fold the fangs back against the roof of the mouth allows vipers to have longer fangs than snakes such as cobras, which have fangs in a fixed position (Nilson and Gutberlet 2004).

Distribution and habitat

Vipers are found in both the Old and New Worlds. Members of the subfamilies Viperinae, Causinae, and Azemiopinae are found only in the Old World (Nilson and Gutberlet 2004). Viperines are found in Africa, Europe, and Asia, while members of Causus is restricted to sub-Sahara Africa and Azemiops occurs in southern China, Laos, Vietnam, and Myanmar (Nilson and Gutberlet 2004). Crotalines inhabit both the Old and New World. Australia lacks any viperids.

Viperids are found in both temperate and tropical environments and in a wide variety of habitats, including terrestrial and arboreal, and grasslands, dry steppe, mountains, forests, savannas, and so forth.

Compared to many other snakes, vipers often appear rather sluggish. Typically, vipers are nocturnal and ambush their prey, although some may forage actively. Long fangs allow vipers to strike deeply into tissues to cause severe necrosis.

Experiments have shown that vipers are capable of making decisions on how much venom to inject depending on the circumstances. In all cases, the most important determinant of venom expenditure is generally the size of the snake, with larger specimens being capable of delivering much more venom. Also, the species is important, since some are likely to inject more than others. Other factors include how much venom is available, the accuracy of the strike, and the number of bites already delivered in a short space of time.

In predatory bites, factors that influence the amount of venom injected include the size of the prey, the species of prey, and whether the prey item is held or released. The need to label prey for chemosensory relocation after a bite and release may also play a role. In defensive bites, the amount of venom injected may be determined by the size or species of the predator (or antagonist), as well as the assessed level of threat, although larger assailants and higher threat levels may not necessarily lead to larger amounts of venom being injected (Hayes et al. 2002).

Most viperids are ovoviviparous, giving birth to live young, but a few lay eggs. The word "viper" is derived from Latin vivo for "I live," and pario for "I give birth" (Schuett et al. 2002).

Venom

Viperid venoms typically contain an abundance of protein-degrading enzymes, called proteases, that produce symptoms such as pain, strong local swelling and necrosis, blood loss from cardiovascular damage complicated by coagulopathy, and disruption of the blood clotting system. Death is usually caused by collapse in blood pressure. This is in contrast to elapid venoms that generally contain neurotoxins that disable muscle contraction and cause paralysis. Death from elapid bites usually results from asphyxiation because the diaphragm can no longer contract. However, this rule does not always apply: some elapid bites include proteolytic symptoms typical of viperid bites, while some viperid bites produce neurotoxic symptoms (Slowinski 2000).

Proteolytic venom is also dual-purpose: it is used for defense and to immobilize prey, as with neurotoxic venoms, and also many of the enzymes have a digestive function, breaking down molecules in prey items, such as lipids, nucleic acids, and proteins (Slowinski 2000). This is important, as many vipers have weak digestive systems (Smith 2004).

Due to the nature of proteolytic venom, a viperid bite is often a very painful experience and should always be taken seriously, even though it is not necessarily fatal. Even with prompt and proper treatment, a bite can still result in a permanent scar, and in the worst cases the affected limb may even have to be amputated. A victim's fate is impossible to predict as this depends on many factors, including (but not limited to) the species and size of the snake involved, how much venom was injected (if any), and the size and condition of the patient before being bitten. The patient may also be allergic to the venom and/or the antivenin.

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